Field
The present application discloses various aspects relating to eddy current detection technology.
Related Art
In general, materials may have defects (or flaws) in them, such as cracks, inclusions and corrosion. The defects may form for various reasons, including as a result of manufacturing and/or stresses experienced by the material over its lifetime.
One manner for detecting such defects in a conductive material (such as a metal or metal alloy) is to generate eddy currents within the material and detect the resulting magnetic fields. Eddy currents are generated in a conductive material in response to a suitable time varying magnetic field being applied to the conductive material. The time varying magnetic field gives rise to a force on the electrons in the conductive material, thus creating current, referred to as “eddy current.” The eddy currents themselves give rise to magnetic fields, referred to as induced magnetic fields, which oppose the incident magnetic field. The distributions of the eddy currents will be altered by cracks (or other defects) in the material, thus creating perturbations in the induced magnetic fields. The changes in the induced magnetic fields, which are detected with an eddy current probe, give an indication of the presence of the cracks (or other defects) and their characteristics (e.g., location, size, shape, etc.). Generally, the magnetic field due to the coil as well as the magnetic fields arising from the eddy currents induced in a uniform material have a well characterized spatial distribution which is exactly axial at the center of the current loop and has field lines that surround the current distribution. The magnetic fields has both radial and axial components, and near the center the in plane component is very small. For a circular coil and a uniform material, the tangential component of both direct and induced magnetic fields is zero. In contrast, if there are cracks or other irregularities in the material which disrupt the eddy currents and perturb the magnetic field, the induced magnetic field may be modified and may have substantial in-plane components and possibly substantial tangential components. These in-plane components may be easier to detect than changes in the substantial axial magnetic field.
This effect may be especially pronounced if the position of the crack or flaw relative to the coil is such that the maximum eddy current density would pass through that point in the absence of the flaw, and if the characteristic depth of the eddy current distribution is comparable to or smaller than the extent of the crack or flaw in the depth direction.
Conventional coil-based eddy current probes generally take one of two forms. A first type of conventional coil-based eddy current probe uses a single coil (i.e., a combined drive/detection coil) to both carry the current that generates (or drives) the incident magnetic field applied to the conductive material under test and detect the magnetic field due to the eddy currents in the material under test. Monitoring this field allows the instrument to detect changes caused by cracks or other flaws. A second type of conventional coil-based eddy current probe uses two distinct co-axial coils—one which carries the current that generates (or drives) the incident magnetic field applied to the conductive material under test and a second which detects the total magnetic field and can be monitored to detect changes due to cracks (or other defects) in the material under test.
FIG. 1 illustrates a conventional coil-based eddy current probe of the first type. The probe 100 includes a single coil 102 through which an alternating current (AC) current is applied to generate a magnetic field incident upon a conductive material under test 104 when the probe is placed in proximity to the material under test. The incident magnetic field gives rise to eddy currents in the material under test 104 as shown which generate a magnetic flux which passes through the coil 102. A crack (or other type of defect) 106 in the material under test 104 disturbs the eddy currents 108 and therefore the magnetic flux. The disturbance in the magnetic flux thus indicates the presence of the crack (or other type of defect).
FIG. 2 illustrates a conventional coil-based eddy current probe of the second type. As shown, the probe 200 includes two distinct but co-axial coils, a drive coil 202 to generate the eddy currents in the material under test by applying an incident magnetic field and a detection coil 204 (of one or more turns) to detect the magnetic flux resulting from the eddy current. Because the coils 202 and 204 are co-axial, the sensitive axis of the detection coil is parallel to the principal axis of the drive coil (i.e., the primary direction of the magnetic field generated by the drive coil).
It should be appreciated from FIGS. 1 and 2 that both of these types of conventional coil-based eddy current probes use a detection coil that is sensitive to the magnetic field components oriented in the same direction as the magnetic fields created by the drive coil. These fields are generally oriented in the direction normal to the surface of the material under test.
In the case of a two coil eddy current sensor, however, the detection coil may alternately be arranged with its axis at an angle to the drive coil axis, so as to be more sensitive to in plane components of the magnetic field or specifically to the tangential direction or to in-plane components (either tangential or radial) at the center of the coil, or to reduce its sensitivity to the out-of-plane component of the magnetic field.
Some conventional eddy current probes do not use a detection coil, and instead use a solid-state magnetic field detecting element. These include magneto-resistive sensors (such as anisotropic (AMR) or giant magnetoresistive sensors), Hall Effect sensors, and superconducting quantum interference devices (SQUIDS). In the case of magnetoresistive sensors, the resistance of the sensor varies depending on the magnetic field applied to the sensor. Thus, when an AMR sensor is placed in the presence of an eddy current, the magnetic fields generated by the eddy current may alter the resistance value of the AMR sensor. The alteration in the resistance value is used to detect the presence and strength of the eddy currents and thus of any defects in the material under test.